Optocouplers are used in Safe Torque Off (STO) circuits for cost and electrical safety reasons. Typically, the STO is rarely triggered in systems with low security requirements. That is, a primary side (light emitting diode, LED) of the optocoupler is in an always-ON state for an extremely long period of time. This will seriously reduce the lifetime of the optocoupler. In addition, since there is almost no chance to turn off the LED of the optocoupler for effective diagnosis, a stuck-at fault may occur and safety may be affected.
In general, the reliability of the optocoupler is not very high (compared to other types of isolating components). Therefore, in systems with high safety requirements, it is important to diagnose whether the optocoupler is operating properly. A conventional approach is to add an inversed optocoupler in a STO channel (typically, for high reliability/safety, a redundant configuration is employed, for example, two STO channels are used) to trigger an online dynamic check signal. This leads to an increase in cost and circuit/software complexity. Another approach is to use a dynamic power source in the STO channel, but this requires to add a transformer, which will occupy a lot of space.